1D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

Can Small, Fast Spinning Asteroids be Rubble Piles?

D.J. Scheeres and P. Sánchez

Department of Aerospace Engineering SciencesThe University of Colorado Boulder

Research support from NASA’s NEOO and PG&G programs is acknowledged

2D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

What is a Rubble Pile?

• A size distribution of boulders and grains.– Extends from ~ microns to several decameters

– Measurements of Itokawa suggest cumulative size distributions of:• ~1/d2.8 from ~ millimeters to decameters

• ~1/d2.8 from microns to 100 microns

• Small regolith “dominates” in surface area, but not in volume– Implies that larger boulders and grains are coated in a matrix of finer grains

• What are the consequences of this?– The fine grains can serve as a “cement” that holds larger blocks in place.

– Allows us to apply basic properties of cohesive grains measured on Earth and the Moon to provide predictions for cohesive strength of a rubble pile.

3D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

Example Computation

• Cohesive forces can strengthen rubble piles, allowing them to spin faster – 1-meter boulders with cohesive interstitial regolith van der Waals forces

– Equal pull forces applied to each... very different outcomes

Cohesionless regolith Cohesive regolith

4D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

How Fast can a Cohesive Rubble Pile Spin?

Strong Lunar Regolith Cohesion: 3 kPa

Weak Lunar Regolith Cohesion: 100 Pa

“Very Weak” Cohesion: 25 Pa

Monoliths?

Rubble Piles?

2008 TC3 = Almahata Sitta Meteorites: Could have been a tumbling rubble pile!

5D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

How Fast Must a Boulder Spin to Clear Grains?

Millimeter-sized grainsCentimeter-sized grainsDecimeter-sized grains

100 Micron-sized grains

10 Micron-sized grains

Even Fast-spinning “monoliths” can be covered with finer-grained regolith.

Strength based on lunar regolith cohesion

6D.J. Scheeres, A. Richard Seebass Chair, University of Colorado at Boulder

Summary on Small Body Cohesion:Strength and Surfaces

• Rubble pile asteroids can be strengthened by cohesive forces between their smallest grains

• Cohesive strength less than found in the upper lunar regolith can allow ~10 meter rubble piles to spin with periods less than 5 minutes!

• “Monolithic boulders” ~ 10 meters in size and spinning with periods much faster than ~ 1 minute can have millimeter to micron grains on their surfaces.